Air partial expansion refrigeration for cryogenic air separation

Abstract

The invention provides an improved means of producing the refrigeration required for any fractional distillation air separation process. The improved refrigeration technique causes the distillation columns to operate more efficiently, and thereby permits increased recovery and/or purity of product at lower energy input. Referring to FIG. 3, refrigeration air is partially expanded in expander 313, then condensed by exchanging latent heat with depressurized kettle liquid in condenser 314, and the resulting liquid air is split and used to reflux both columns 307 and 305 via pump 316 and valve 315 respectively.

I claim: 
     
       1. Process for cryogenic distillation of compressed air to oxygen product plus optional crude argon byproduct comprising: (a) supplying an uncondensed fraction of the supply air to a high pressure (HP) rectifier;   (b) withdrawing overhead liquid from the HP rectifier and feeding at least part of it to a low pressure nitrogen removal column as overheat reflux therefor;   (c) work expanding a minor fraction of the supply air to an intermediate pressure;   (d) condensing the expanded air by exchanging latent heat with at least one of N 2  removal column intermediate height liquid and at least part of the HP rectifier bottom liquid (kettle liquid); and   (e) splitting the resulting liquid air into at least two fractions and feeding one fraction to an intermediate reflux height of the HP rectifier and another to the N 2  removal column.   
     
     
       2. Process according to claim 1 further comprising directly supplying a major fraction of the supply air to the HP rectifier without preliminary partial condensation, and further comprising: (a) increasing the pressure of one of said fractions of liquid air before intermediate refluxing the HP rectifier with it;   (b) additionally compressing the minor fraction of air to be work-expanded prior to said work expansion; and   (c) powering said compression step with the work produced in said expansion step.   
     
     
       3. Process according to claim 1 further comprising: (a) depressurizing kettle liquid to the approximate pressure of said N 2  rejection column;   (b) supplying said depressurized kettle liquid to said air condensing step;   (c) partially evaporating said kettle liquid by exchanging latent heat with condensing air;   (d) feeding at least the vapor fraction of the partially evaporated kettle liquid to the N 2  removal column;   (e) condensing partially work-expanded air by said latent heat exchanging step in an amount which is between about 10 and 24 percent of the compressed air supply; and   (f) supplying separate liquid air intermediate reflux stream to the HP rectifier and N 2  rejection column which are each between about 5 and 12 percent of the compressed air supply.   
     
     
       4. Process according to claim 1 further comprising reboiling N 2  rejection column intermediate height liquid from the same equilibrium stage as the optionally evaporated kettle liquid feed height by said latent heat exchange with expanded air. 
     
     
       5. Process according to claim 1 further comprising: (a) additionally compressing the minor fraction of supply air to be work expanded at least once prior to said work expansion;   (b) cooling said minor fraction after compressing but before expanding; and   (c) powering one of said additional compressions from work developed by said expansion and another by an external power source.   
     
     
       6. Process according to claim 1 further comprising: (a) partially condensing part of said compressed air prior to said supplying of the uncondensed fraction to the HP rectifier;   (b) increasing the pressure of one of said fractions of liquid air before said feeding of it to the HP rectifier;   (c) evaporating liquid oxygen bottom product from said N 2  rejection column by exchanging latent heat with said partially condensing air; and   (d) providing at least one of N 2  rejection column bottoms reboil and gaseous oxygen product from said evaporating liquid oxygen.   
     
     
       7. Process according to claim 6 further comprising: (a) additionally compressing the minor fraction of air to be work expanded prior to said work expansion;   (b) powering said compression with the work produced by said expansion; and   (c) exchanging latent heat between HP rectifier overhead vapor and N 2  rejection column intermediate height liquid, thereby providing overhead reflux to the HP rectifier and intermediate reboil to the N 2  rejection column.   
     
     
       8. Process according to claim 6 further comprising: (a) reboiling the bottom of the N 2  rejection column by exchanging latent heat with HP rectifier overhead vapor;   (b) providing an argon sidearm column in vapor and liquid communication with the N 2  rejection column;   (c) withdrawing crude argon sidearm; and   (d) increasing the pressure of the liquid oxygen bottom product from the N 2  rejection column prior to evaporating it by latent heat exchange with partially condensing supply air.   
     
     
       9. Process according to claim 8 further comprising: (a) locating the liquid oxygen evaporator at a lower elevation than the bottom of the N 2  rejection column whereby at least part of the said pressure increase is obtained by the hydrostatic head of the liquid oxygen;   (b) at least partially evaporating depressurized kettle liquid in at least two sequential stages by exchanging latent heat with argon sidearm column vapor, thereby providing argon sidearm column reflux and at least two vapor streams of differing composition; and   (c) feeding said two vapor streams to different heights of said N 2  rejection column.   
     
     
       10. Process according to claim 6 further comprising: (a) providing an argon distillation column which is fed a liquid oxygen-argon mixture from the N 2  rejection column; and   (b) reboiling the argon column by exchanging latent heat with HP rectifier overhead vapor.   
     
     
       11. Process according to claim 10 further comprising: (a) additionally compressing the minor fraction of air to be work-expanded prior to said work expansion;   (b) powering said compression with the work produced by said expansion;   (c) refluxing the argon column overhead by exchanging latent heat with partially evaporated depressurized kettle liquid;   (d) refluxing an intermediate height of the argon column by exchanging latent heat with the remaining unevaporated kettle liquid; and   (e) feeding the vapor streams from steps (c) and (d) to different heights of the N 2  rejection column.   
     
     
       12. Process according to claim 10 further comprising (a) additionally compressing the minor fraction of air to be work-expanded prior to said work expansion;   (b) powering said compression with an external source of power;   (c) selecting the N 2  rejection column feed location as the height from which intermediate height liquid is obtained for latent heat exchange with expanded air; and   (d) evaporating at least a major fraction of the gaseous oxygen product by said step of exchanging latent heat with partially condensing air.   
     
     
       13. In a process for separating air into valuable products by cryogenic fractional distillation comprising rectifying supply air in a high pressure column; distilling depressurized and optionally evaporated bottom liquid from the high pressure column into at least overhead N 2  and bottoms O 2  in a low pressure column; and refluxing the low pressure column by injecting liquid N 2  overhead product from the HP column into the LP column overhead; the improvement comprising producing refrigeration for said air separation process by: (a) work expanding a minor fraction of said supply air;   (b) condensing said expanded air by exchanging latent heat with at least one of low pressure column intermediate height liquid and depressurized high pressure column bottom liquid;   (c) splitting the condensed air into two fractions;   (d) feeding one fraction to a higher intermediate height of the low pressure column as intermediate reflux therefor; and   (e) increasing the pressure of the remaining fraction of condensed air and feeding it to an intermediate height of the high pressure column as intermediate reflux therefor.   
     
     
       14. A process for producing at least one of oxygen, nitrogen, and coproduct crude argon from compressed air by fractional distillation comprising: (a) rectifying an uncondensed major fraction of the compressed air in a high pressure rectifier to liquid nitrogen overhead product and oxygen enriched liquid bottom product (kettle liquid);   (b) additionally compressing a minor fraction of said compressed air;   (c) work expanding said fraction of the compressed air to an intermediate pressure;   (d) powering said additional compression by the work obtained from said expansion;   (e) condensing the expanded air by exchanging latent heat with depressurized kettle liquid, thereby partly evaporating said kettle liquid;   (f) distilling the at least partly evaporated kettle liquid to gaseous overhead N 2  and fluid O 2  bottom product in a low pressure distillation column (LP column);   (g) splitting said condensed air into two approximately equal fractions;   (h) supplying one fraction to an intermediate height of the low pressure column; and   (i) increasing the pressure of the remaining fraction and supplying it to an intermediate height of the high pressure rectifier.   
     
     
       15. Process according to claim 14 further comprising: (a) partially condensing said compressed air prior to said supplying of the uncondensed major fraction to the HP rectifier; and   (b) evaporating liquid oxygen bottom product from said LP column by exchanging latent heat with the partially condensing compressed air; and   (c) providing at least one of LP column bottoms reboil and gaseous oxygen product from said evaporating liquid oxygen.   
     
     
       16. Apparatus for cryogenic fractional distillation of compressed air comprising: (a) a high pressure column which is supplied an uncondensed major fraction of said compressed air;   (b) a lower pressure column which is refluxed with liquid N 2  from the overheat product of said HP rectifier;   (c) a work expander which is supplied a minor fraction of said compressed air after partial cooling;   (d) at least one latent heat exchanger which is supplied expanded air from said work expander and one of depressurized kettle liquid and LP column feed height liquid;   (e) means for splitting the condensed air from said latent heat exchanger into two fractions;   (f) means for supplying one of said liquid fractions to an intermediate height of the LP column; and   (g) means for pressurizing the remaining fraction and supplying it to an intermediate height of said HP column.   
     
     
       17. Apparatus according to claim 16 wherein said means for pressurization is a liquid pump and further comprising: (a) latent heat exchanger for evaporating liquid oxygen bottom product from said LP column via partial condensation of said major air fraction; and   (b) means for compressing said minor fraction of air prior to said expansion, said means for compressing being powered by said work expander.